Wikipedia
reCAPTCHA is a CAPTCHA-like system designed to establish that a computer user is human (normally in order to protect websites from bots) and, at the same time, assist in the digitization of books. The reCAPTCHA service supplies subscribing websites with images of words that are hard to read for optical character recognition (OCR) software. The subscribing websites (whose purposes are generally unrelated to the book digitization project) present these images for humans to decipher as CAPTCHA words, as part of their normal validation procedures. They then return the results to the reCAPTCHA service, which sends the results to the digitization projects.
reCAPTCHA was originally developed by Luis von Ahn, Ben Maurer, Colin McMillen, David Abraham and Manuel Blum at Carnegie Mellon University's main Pittsburgh campus. It was acquired by Google in September 2009.
reCAPTCHA has completed digitizing the archives of The New York Times and books from Google Books. The archive can be searched from the New York Times Article Archive, where more than 13 million articles in total have been archived, dating from 1851 to the present day.
The system has been reported as displaying over 100 million CAPTCHAs every day, on sites such as Facebook, TicketMaster, Twitter, 4chan, CNN.com, StumbleUpon, Craigslist (since June 2008), and the U.S. National Telecommunications and Information Administration's digital TV converter box coupon program website (as part of the US DTV transition).
reCAPTCHA's slogan was "Stop Spam, Read Books.", until the introduction of a new version of the reCAPTCHA plugin in 2014; the slogan has now disappeared from the website and from the classic version of the reCAPTCHA plugin. A new system featuring image verification was also introduced. In this system, users are asked to just click on a checkbox (the system will verify whether the user is a human or not, for example, with some clues such as already-known cookies or mouse movements within the ReCAPTCHA frame) or, if it fails, select one or more images from a selection of nine images.
Through mass-collaboration, reCAPTCHA is helping to digitize books that are too illegible to be scanned by computers, as well as translate books to different languages.